1. Field of the Invention
The present invention relates to the field of input signal interface circuits. More particularly, the present invention relates to an input interface circuit useful for controlling input signal lines within an integrated circuit device.
2. Background of the Invention
Signals are often carried to an integrated circuit (IC) via an input pin of the IC which is also coupled to an external pad within the IC. Other signals are carried within an IC from one portion of the IC to another portion of the IC; these signals can also be called input signals. A significant problem exists with present circuit designs regarding input signal lines to an IC when the input signal characteristics (e.g., voltage, driving source) are unknown. With respect to complementary metal oxide semiconductor (CMOS) technology, for example, the voltage applied to an input pin over a bus is in many instances unknown because the source driver can be switched during operation, removed completely, or in a power conservation mode. In these instances when no source device is driving the input pin, the voltage seen by the integrated circuit over its associated external pad may be unknown and may be driven with a very weak high or weak low signal or may simply "float." Likewise, certain internal lines in an IC can float to an unknown voltage. In such conditions, there may be amplification of ambient noise, as well as excessive power dissipation associated with the input pin of the IC device or with an internal line of the IC device. Therefore, it is desired to provide a known or predicted voltage that can be applied to each input pin of an IC device even during conditions when the connected bus does not have a driving source or the bus is disconnected or electronically isolated. To this end, it is also desired to provide flexibility in generating signal outputs from an interface circuit based on signals received over an input pin that may not have a driving source. The present invention offers this functionality.
One method of preventing input pins or internal lines from floating at unknown or unstable voltages, caused by leakage currents on a line, is to provide a pull-up or pull-down resistance. Using such mechanisms, in the event that the normal driving source is mechanically disconnected or electrically not driven, as in a tri-state bus, the resistor provides a defined level for the input pin. For tri-state buses, use of pull-up or pull-down resistors may result in unnecessarily changing the logic level of many of the lines as the bus changes from one driving source to another.
The use of a circuit to minimize floating by retaining the last voltage applied is also known in the art. Such circuits are often referred to as "keeper circuits". FIG. 1 is a schematic diagram of a prior art keeper circuit 90. With respect to this prior art device, a pair of inverters 100 communicate an input signal 102 through a feedback impedance 104, which in this example is a resistor 104. The signal then reinforces the logic value.
The output state of the prior art circuit 90 of FIG. 1 is therefore determined by the last state of the input signal at an input terminal 98, and this state is maintained by the keeper circuit if the normal source is removed or if the normal source becomes tri-stated. An amplifier/buffer circuit 99 communicates the `kept` state to a logic circuit which, in this example, is a logic array.
Signal interface circuitry has been known to utilize an interface that contains an active keeper circuit in conjunction with a pull-up transistor where the pull-up transistor can be programmed out of the interface and where the keeper circuit is always present. This design does not offer as much flexibility as desired because the keeper circuit is always active in the interface and only the pull-up transistor can be programmed in or out. Further, this design has only been applied to internal lines of a logic device and has not been applied to external pads of the IC (e.g., those coupled to receive signals from the input pins of the IC).
In addition, pull-up transistors and pull-down transistors have been applied to external pads of a logic device. However only one of the transistors is allowed to be programmed permanently "on" at a time, while both can be programmed permanently "off." This design does not offer as much flexibility as desired because the pull-up and pull-down devices are limited in their configuration with respect to each other and are programmed only in permanent states. Further, in this application there is no keeper circuit function.
Accordingly, it is desired to provide an input interface circuit having the flexibility to independently control a set of pull-up and pull-down transistors such that multiple configurations can be obtained, including a keeper arrangement. It is further desired to apply such interface circuit to an external pad of an IC device which is coupled to receive signals from an input pin. It is also desired to provide the above flexibility such that each transistor in the above configuration can be independently programmed permanently off, or programmed permanently on, or programmed to follow the input voltage, or follow the inverse of the input voltage. It is desired to provide this interface circuit for each external pad of the IC device using circuitry that is internal to the IC device. These and other aspects of the present invention not specifically recited above will become clear within discussions of the present invention to follow.